In U.S. Pat. No. 4,010,139 there are described polyolefinic compositions, suited for the manufacture of self-extinguishing articles, comprising:
(1) polypropylene;
(2) a mixture of (a) basic bismuth carbonate (BiO).sub.2 CO.sub.3 and (b) a chlorinated paraffin wax; component (a) being present in such an amount as to have a bismuth content in the composition comprised between 0.5 and 5% by weight, and component (b) being present in an amount comprised between 1% and 10% by weight of the composition.
Compositions of this type, comprising 5% of a chlorinated paraffin wax at 70% of Cl and 1.65% of (BiO).sub.2 CO.sub.3 (corresponding to 1.35% of Bi metal), display excellent flame resistance characteristics which correspond to an oxygen index equal to 29 (the oxygen index expresses the minimum oxygen percentage in an oxygen/nitrogen mixture, necessary for a test piece to burn in a continuous way).
In U.S. patent application Ser. No. 821,844 filed Aug. 4, 1977 there are described bumpers for motor vehicles, manufactured from an intimate mixture of
(a) 95-80% by weight of a thermoplastic block copolymer propylene-ethylene containing from 3% to 20% by weight of copolymerized ethylene and having a flow index, measured at 230.degree. C. according to ASTM D 1238 condition L standards; comprised between 0.2 and 1, and an elasticity modulus at 23.degree. C., measured according to ASTM D 790 standards, of at least 1100 MN/m.sup.2 ;
(b) from 5 to 20% by weight of an ethylene-propylene elastomeric copolymer containing 50-70% by weight of copolymerized ethylene and having a Mooney viscosity ML (1+4) at 100.degree. C. comprised between 50 and 100;
(c) possibly inert, powder mineral fillers.
By the term "thermoplastic block copolymers" are meant those polymeric products which are obtained by polymerizing propylene in the presence of Ziegler-Natta catalysts, which polymerization is successively continued with the addition of ethylene or ethylene/propylene mixtures; said polymeric products are briefly described as "polypropylene modified with small percentages of copolymerized ethylene", and may be prepared according to processes described, for instance, in British Pat. No. 915,622 and in U.S. Pat. No. 3,629,368.
With the above described polymeric compositions it is possible to manufacture bumpers for motor vehicles having an elasticity modulus sufficiently high throughout a wide temperature range, particularly from -30.degree. C. to +60.degree. C., and displaying a high resistance to embrittlement at low temperatures and, consequently, a better impact resistance.
In order to impart to said shock resistant polymeric compositions flame-resistant properties, into those compositions there were incorporated the same additives already used with success in the case of polypropylene and which have been previously described. The result was that, while the anti-impact properties of the compositions remain unaltered, the self-extinguishing characteristics attain a level more or less inferior to that which is attained incorporating the additives into the pure polypropylene.
From TABLE I it is possible to see that the oxygen index of shock resistant compositions decreases as the percentage of ethylene-propylene elastomer present in the mixture increases; with a percentage of 20% of elastomer, the oxygen index drops to a value of 23. From that same Table it will also be learned that the oxygen index is further reduced when using greater amounts of the two additives.
In the tests recorded on Table I, as thermoplastic block copolymer, there was used a propylene-ethylene copolymer containing 7% by weight of copolymerized ethylene and having a flow index, measured as already indicated, equal to 1.
The elastomer added to said copolymer in ever increasing amounts is an elastomeric ethylene-propylene copolymer containing 55% by weight of copolymerized ethylene and having a Mooney viscosity ML (1+4) at 100.degree. C., equal to 89.
For sake of shortness, a mixture of the thermoplastic copolymer with elastomer, having a certain content of the latter, was simply indicated by the term "blend".
In order to prepare the mixtures indicated on the tables, the thermoplastic copolymer and the elastomer are mixed together in the molten state, preferably in the presence of antioxidants and/or thermal stabilizers, using the methods and equipments conventionally used in the processing of polyolefinic plastic materials, such as for instance wormscrew mixers or rotary mixers of the Banbury type, suited for producing perfectly homogeneous blends.
To the molten mass are added the various additives and the mix thus obtained is extruded in a suitable extruder and converted to granules.
With the granular product 3 mm thick plates are molded on a small CARVER type plate press, operating for 7 minutes at 200.degree. C. and at a pressure of 40 kg/sq.cm.
On the plates thus prepared, it is possible to determine the oxygen index and possibly other properties, such as for instance the glass transition temperature (Tg).
TABLE I __________________________________________________________________________ Parts by Weight COMPONENTS Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. __________________________________________________________________________ 11 Polypropylene 100 93.95 92.85 -- -- -- -- -- -- -- -- Block thermoplastic co- polymer -- -- -- 100 93.35 92.85 -- -- -- -- -- Blend at 5% of elastomer -- -- -- -- -- -- 92.85 -- -- -- -- Blend at 10% of elastomer -- -- -- -- -- -- -- 92.85 -- -- -- Blend at 15% of elastomer -- -- -- -- -- -- -- -- 92.85 -- -- Blend at 20% of elastomer -- -- -- -- -- -- -- -- -- 92.85 -- Blend at 17.5% of elastomer -- -- -- -- -- -- -- -- -- -- 79.5 (BiO).sub.2 CO.sub.3 -- 1.65 1.65 -- 1.65 1.65 1.65 1.65 1.65 1.65 5 "Cereclor 70"* -- 5 5 -- 5 5 5 5 5 5 15 "Irgastab T 290"** -- -- 0.5 -- -- 0.5 0.5 0.5 0.5 0.5 0.5 Glass transition tempera- ture (Tg).sup.1 -- -- -- -- -- -- -10.degree. C. -26.degree. C. -40.degree. C. -50.degree. C. -30.degree. C. Oxygen Index.sup.2 17 29 28 17 27 26.5 26 25 24.5 23 22 __________________________________________________________________________ *Chlorinated paraffin wax at 70% of Cl (ICI) **Dibutyl tin maleate .sup.1 Measurement carried out according to BS 2782306B standards .sup.2 Measurements carried out according to ASTM D2863 standards